As it is known, some internal combustion engines are provided with a turbocharger supercharging system, which is capable of increasing the power developed by the engine by exploiting the enthalpy of the exhaust gases for compressing the air taken in by the engine and thus increasing the volumetric intake efficiency.
A turbocharger supercharging system comprises a turbocharger provided with a turbine, which is arranged along an exhaust pipe to rotate at a high speed under the pressure of the exhaust gases expelled by the engine, and with a compressor, which is made to rotate by the turbine and is arranged along the air feeding pipe to compress the air taken in by the engine.
In a turbocharger supercharging system the operating field of the turbocharger must be kept within a useful zone depending on the crank position both for functional reasons (i.e. to avoid irregular or in any case low efficiency operation) and for structural reasons (i.e. to avoid damage to the turbocharger). Specifically, on the left side of the Reduced Mass Flow Rate/Compression Ratio map there is a “forbidden” zone delimited by the surge line, constituted by the set of points in which the aerodynamic equilibrium inside the compressor is interrupted and a periodic, noisy and violent rejection of flow rate to the mouth occurs, with effects which may be destructive for the blading; instead, on the right side of the Reduced Mass Flow Rate/Compression Ratio map there is a second “forbidden” zone delimited by the so-called “stalling line”, which corresponds to the reaching of chocking conditions (and consequent flow rate stoppage) at the turbine inlet and defines the maximum possible flow rate that the compressor may supply in the given intake environment conditions.
Patent application EP1741895A1 discloses a control method for a turbocharger supercharged internal combustion engine comprising a compressor, a turbine adapted to rotably feed the compressor under the pressure of the engine exhaust gases, and a wastegate valve adapted to adjust the flow rate of exhaust gases input to the turbine for controlling the rotation speed of the turbine itself according to a supercharging pressure target required at the compressor outlet. The control method disclosed in patent application EP1741895A1 comprises the steps of measuring the air pressure taken in at the compressor inlet; determining the mass flow rate of the compressor; calculating a supercharging limit pressure, which is correlated to the pressure of the air obtainable at the compressor outlet when the turbine rotates at a speed essentially equal to the predetermined limit speed by means of a predetermined map which characterizes the operation of the compressor, and according to the predetermined rotation limit speed, the measured air pressure and the mass flow rate; verifying whether a required supercharging pressure target satisfies a predetermined relation with the calculated supercharging limit pressure; if the relation is satisfied, actuating the wastegate valve for controlling the rotation speed of the turbine according to the limit supercharging pressure so as to limit the rotation speed of the turbocharger to a value essentially equal to the predetermined limit speed.